The Central Nervous System is made of all the nervous tissues which form the encephalon and the spinal cord. It is made of white and grey matter. Nerves are excluded.
These noble tissues are essential to the human being functioning, as the encephalon ensures control over the whole body, but they are fragile and must be protected.
The Central Nervous System is protected, from the outside to the inside by:
the cranium bones for the encephalon and the spinal canal for the spinal cord, which act like a “helmet”;
the meninges, which act like «a protective wrapping»;
- the dura mater, which is an outside layer ; - the arachnoid ; - the pia mater, which is directly in contact with the central nervous system.
the CSF or Cerebro-Spinal Fluid, which plays the role of a shock absorber.
Cerebro-spinal fluid is the transparent "gin clear" fluid that fills the cerebral ventricles. It bathes the brain and the spinal cord.
In addition to its mechanical protective role, CSF takes part in metabolic exchange between the nervous system and the rest of the body.
Roles of CSF are:
Nutrition of brain cells;
Cleaning of waste due to cellular metabolism;
Absorption of shocks for the protection of the Central Nervous System.
CSF is produced in the ventricles by the choroid plexus at a rate of approximately 20 ml/h in adults (8 ml/h in infants).
The CSF circulates within the lateral ventricle, the 3rd ventricle and leave the 4th ventricle through the foramina, entering the sub-arachnoid spaces surrounding the brain and spinal cord. These sub-arachnoid spaces are fibrous tissues situated between the pia mater and the arachnoid, which behave like a sponge. Ventricles and foramina are cavities that have a role of reservoir for the CSF. There is permanently 125 ml to 150 ml of CSF in the ventricles.
CSF pathways
The fluid returns to the venous circulation by absorption through small formations, the arachnoid villi (Pacchioni granules), a type of outgrowth from the arachnoid, through the dura mater and spaces in contact with the sagittal sinus at the midline of the brain.
Meninges and CSF absorption in sagittal sinus
With this cycle of production, circulation and absorption, under normal conditions, a perfect equilibrium exists between secretion and absorption of CSF.
1. The brain, i.e. the parenchymal compartment; 2. The blood, i.e. the vascular compartment; 3. The Cerebro-Spinal Fluid, i.e. the liquid compartment.
As the cranium is an inextensible cavity for the adult, Intra-Cranial Pressure is then the combination of the pressures exerted by these three components. If the volume of one of those three components increases, the volume of one of the two others at least must decrease, or the Intra-Cranial Pressure will increase as a mechanical consequence.
Potential causes for an increase in volume are:
for the brain: a tumor, an oedema, an abcess...
for the blood: an hematoma or a brain haemorrage;
for the CSF: hydrocephalus (= excess of liquid in the brain).
Cerebral compliance corresponds to the « elasticity » of the brain. The brain has the capacity to behave like a sponge which will absorb a variation in volume of one of the compartment without increasing pressure. In other words, compliance is the capacity of the three brain components (cerebral tissues, CSF, blood) to maintain a normal ICP. The better the compliance is, the lower the pressure variation for a given variation in volume and the lower the pathology progression. Compliance decreases with age.
Cerebral compliance allows a compensation phase during which brain adapts itself to variations of volume in one or the other of the compartments. For instance, blood volume can vary by distension or contraction of blood vessels, and volume of CSF by the regulation of its production and re-absorption cycle.
Cerebral compliance has its limits though. After some time, the de-compensation phase is reached, which provokes a rapid increase in ICP, with serious complications for the patient. ICP monitoring is used to avoid the switch from the compensation phase to the de-compensation phase.
II - Normal vs pathological values of ICP:
The reference measurement of CSF pressure is measured at the level of the cerebral ventricles: this is the intra-ventricular pressure. It is often also called intra-cranial pressure (ICP).
The figures hereafter are indicative values which show that ICP is lower among children than among adults. Indeed, in the infant, since the cranium bones are not knit together, a deformation is still possible, that will compensate part of the variation in volume of one of the compartment avoiding an increase in pressure.
Symptoms of a rise in ICP are headaches, vomiting, visual troubles, consciousness troubles.
Normal ICP value (adult lying on his side) is comprised between 6 and 15 mmHg. The pressure is almost zero or even negative when standing up.
Above 20 mmHg, the situation is pathological.
Between 15 and 20 mmHg, symptoms will vary among patients. Some will not develop any symptom, even with ICP superior to 15 mmHg. Others will not withstand it and will develop some or all of symptoms described above (headaches, vomiting, visual or consciousness troubles…).
The scheme below give ICP values on two different scales: mmHg and mmH2O. Most of the time, ICP is given in mmHg.
III - Objective of ICP monitoring:
The objective of ICP monitoring is to follow the trend of intracranial pressure, because the pressure values determine what interventions are necessary to avoid any additional cerebral lesion, which could be irreversible and lethal.
ICP monitoring gives access to the Cerebral Pressure Perfusion value, which is very important, because CPP is the pressure allowing a correct brain perfusion, and then a correct oxygenation. Should CPP be insufficient, ischemia would appear, leading to a decrease of blood-brain barrier efficiency. In parallel, excessive ICP can lead to brain herniation.
That is the reason why ICP is monitored to maintain CPP at 60/70 mmHg to avoid to reach the de-compensation phase and to avoid ischemia.
ICP is monitored thanks to a dedicated monitoring system such as the Pressio®, in the Intensive Care Unit of the hospital.
A pressure-sensor tipped catheter is inserted into the brain of the patient, through a small hole in the cranium. The other extremity of the catheter is connected to a monitor, which displays the ICP values. These ICP values have to meet the alarm threshold criteria entered in the system, otherwise the monitor will emit a warning signal.
The Central Nervous System is made of all the nervous tissues which form the encephalon and the spinal cord. It is made of white and grey matter. Nerves are excluded.
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